cmd/compile: ensure temp register mask isn't empty

We need to avoid nospill registers at this point in regalloc.
Make sure that we don't restrict our register set to avoid registers
desired by other instructions, if the resulting set includes only
nospill registers.

Fixes #57846

Change-Id: I05478e4513c484755dc2e8621d73dac868e45a27
Reviewed-on: https://go-review.googlesource.com/c/go/+/461685
Reviewed-by: Keith Randall <[email protected]>
Run-TryBot: Keith Randall <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: randall77

src/cmd/compile/internal/ssa/regalloc.go

@@ -1561,7 +1561,7 @@ func (s *regAllocState) regalloc(f *Func) {
 			// (Not all instructions need that distinct part, but it is conservative.)
 			if opcodeTable[v.Op].needIntTemp {
 				m := s.allocatable & s.f.Config.gpRegMask
-				if m&^desired.avoid != 0 {
+				if m&^desired.avoid&^s.nospill != 0 {
 					m &^= desired.avoid
 				}
 				tmpReg = s.allocReg(m, &tmpVal)

test/fixedbugs/issue57846.go

@@ -0,0 +1,33 @@
+// compile
+
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+func Float64D3(list [][][]float64, value float64) int {
+	valueCount := 0
+	for _, listValue := range list {
+		valueCount += Float64D2(listValue, value)
+	}
+	return valueCount
+}
+
+func Float64(list []float64, value float64) int {
+	valueCount := 0
+	for _, listValue := range list {
+		if listValue == value {
+			valueCount++
+		}
+	}
+	return valueCount
+}
+
+func Float64D2(list [][]float64, value float64) int {
+	valueCount := 0
+	for _, listValue := range list {
+		valueCount += Float64(listValue, value)
+	}
+	return valueCount
+}